Automatic protection switching system in a network

ABSTRACT

A system and method of automatic protection switching for a network ( 100 ) which protect against link or node failure. The protection fibers ( 119, 121 ) in the links are pre-arranged into protection cycles so that if a link failure occurs, protection switches in the network nodes ( 101, 107 ) connected in the failed link will switch the working fiber data path onto an alternate path comprised of protection fibers ( 119, 121 ). Data can then be transmitted through the protection cycle around the fault to reach the node on the other side of the failed link. The same protection fibers ( 119, 121 ) can be used to protect a priority connection against center switch failure in a network. The protection switches in each node ( 101, 107 ) which connect working fibers ( 115, 117 ) to protection fibers ( 119, 121 ) are very fast and their settings do not depend on the state of the network.

The U.S. Government has certain rights in this invention pursuant toaward CDR-8421402 by the National Science Foundation.

FIELD OF THE INVENTION

The present invention relates to automatic protection switching systemsand methods in networks which incorporate any type of switching andtransmission technology.

BACKGROUND OF THE INVENTION

Today's communication systems for delivering data from a source to adestination are typically high-speed, high capacity systems that handlemany different types of user services. One example of such a system isan optical fiber network used for telephone traffic or for other dataexchange. These advanced networks can transport vast amounts ofinformation at any given time. The networks are conventionally made oflinks connected to nodes which route information over the network to adesired destination. Since link failures are common for such networks, afault recovery system is needed to ensure that the data is properlyrouted to its destination in the event of a fault in a link. While nodefailures are less frequent, protection against them is also desirable.

Various recovery schemes in the event of a failed link in a network havebeen proposed but all previous schemes have significant shortcomings.One possible scheme for link failure recovery is a dynamic pathrearrangeable mesh architecture. This recovery architecture reroutestransmission paths around a failure point in real time. Spare capacityin the network is used to restore communication on a transmission pathwhen a failure occurs. A control system optimizes the use of availablespare capacity by having a node at one end of the failed link broadcasta restoration message to all the other nodes in the network. A node withspare capacity then acknowledges the message and establishes analternate link from the source to destination. This dynamic method isvery time consuming and computationally expensive. The dynamic methodhas high costs, slow restoration speed, difficult control requirementsand the need to constantly plan upgrades to restoration capacity whentraffic growth occurs. Additionally, this scheme does not correct fornode failures.

Another proposed recovery scheme is a dedicated facility restorationwhich has dedicated protection links to carry the data if a failureoccurs in a link over the main data path. In a “One plus One” protectionscheme, the traffic from the source is transmitted simultaneously overboth the “main” data path (consisting of links and nodes) and the“backup” data path consisting of a different set of links and nodes. Thedecision to switch between main and backup data paths is made at thereceiving station. In this scheme, fifty percent of the networkequipment will always be in standby mode, either operating as a main orbackup data path. The main disadvantage of this recovery scheme is thatit depends on the connection state of the network. Every time aconnection is made from a source to a destination over a link and nodedata path, a second separate backup link and node data path has to befound to support the main path.

Another proposed dedicated recovery scheme is called the “One for One”protection scheme. This scheme has traffic carried over one pathdesignated the “main” data path with a dormant bearer being the“standby” data path. The main designated data path carries the datauntil a fault occurs. If a failure occurs, the traffic is switched tothe standby data path and remains in that configuration until anotherfailure occurs. Thus fifty percent of the network equipment is always instandby mode. This method is not autonomous but requires end-to-endmessages to be transmitted to signal a failure to the source in order toswitch data paths. These extra signals add significantly to the cost ofoperating the network. This recovery scheme also depends on theconnection state of the network and every time a connection is made froma source to destination, a second link and node standby path separatefrom the main path has to be found.

Yet another possible dedicated recovery scheme is a self healing ring.The self healing ring can be either uni-directional or bi-directional. Auni-directional ring has two paths arranged in a ring, composed of asuccession of alternating links and nodes, with each link containing twocommunication lines carrying traffic in opposite directions. Each pathcarries the same information except that the direction of datapropagation is reversed in the second ring. If a link is cut or failed,the network simply relies on the information propagating in the otherring operating in the opposite direction. The receiver of the data canselect either path to obtain the data. Every node in the network isconnected in the ring. The self healing ring can also be abi-directional ring with four paths, two main or “working” ringsoperating in opposite directions and two protection rings, alsooperating in opposite directions. Each link now has two working linescarrying traffic in opposite directions and two protection linescarrying traffic in opposite directions. Under normal operation (nofailures) each working ring carries a portion of the traffic. If a linkfails (i.e., all four lines fail), protection switching is used toperform a loop-back function, rerouting the traffic that normally wouldhave used the failed link, around the ring in the opposite direction.This architecture requires a protection line for each working line as inthe “one for one” architecture. The main disadvantage of theself-healing ring is that the network topology is limited to a ringincluding all the nodes. In a ring, there is no capacity for expansionof network traffic since the limited capacity must be shared with allthe nodes in the ring. Additionally, a ring can only be protected from asingle node or single link failure in the network.

SUMMARY OF THE INVENTION

The present invention is an automatic protection switching system for anetwork which includes a plurality of network nodes and a plurality oflinks, each link connecting a pair of nodes. Each link is composed of apair of working data conduits (communication lines) carrying traffic inopposite directions. In some cases a link may contain more than one pairof working and/or protection conduits. The network nodes contain centerswitches whose function it is to connect sets of working lines intopaths carrying traffic between end user source and destinationequipment. They also contain protection switches, whose function is toconnect set of protection lines into pre-assigned “protection paths”designed to reroute traffic around a failed link or center switch if afault is detected. The automatic protection system protects against bothlink failures and node failures.

The automatic protection system of the present invention is configuredby first modeling the network as a graph whose vertices correspond tothe net nodes and whose edges correspond to the protection conduits. Thepre-assigned protection paths are associated with “protection cycles”.Each cycle protects a part of the network, and an appropriate set ofprotection cycles can be calculated for any network once its topology isknown, so that all links and network nodes are protected. When a failureis detected, the protection switches associated with the failed link orcenter switch are activated, switching the data that would normally usethe failed element, onto a path derived from the cycle protecting thatelement.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying figures showing a preferred embodiment of theinvention, in which:

FIG. 1A shows a seven node planar network with bi-directional protectionfibers accompanying each bi-directional main fiber;

FIG. 1B shows a switching node from FIG. 1A;

FIG. 2A shows two switching nodes joined by a link before a link failureoccurs;

FIG. 2B shows two switching nodes joined by a link after a link failureoccurs;

FIG. 3 shows a network with pre-assigned protection cycles in accordancethe invention;

FIG. 4 shows the seven node planar network of FIG. 1 with a single linkfailure;

FIG. 5 shows a seven node planar network with two link failures;

FIG. 6A shows a 2×3 protection switch within a node;

FIG. 6B shows a 3×2 protection switch within a node;

FIG. 7 shows a diagram of protection cycles adjacent to a node in anetwork;

FIG. 8 shows a switching node after an optical switch failure within thenode;

FIG. 9 shows a seven node network after a failure in one node; and

FIG. 10 shows a flow chart of the steps for providing an automatedprotection system for a network.

DESCRIPTION OF A PREFERRED EMBODIMENT

Automatic protection switching (“APS”) for link failures requires rapidrecovery from any failure of a network link. Links may be made up of oneor more data conduits for transmitting data (including information inany form). In this preferred embodiment, optical fibers will bedescribed as the conduits, so that a link will be composed of pairs offibers carrying traffic in opposite directions. A network node is anystation containing switching equipment, (including protection switches)connected to one or more links and possibly end user equipment, for thepurpose of creating data paths between end users over successions oflinks and intermediate nodes.

A network contains two or more nodes, some or all of which areselectively connected together by links in a particular configuration.In this preferred embodiment, each link in a network contains a numberof optical fibers, each of which carries data in a single directionalong the length of the link. The fibers could also be any conventionaltype of transmission line or data conduit. Each fiber is oriented tocarry data in a single direction by terminating connections and/or otherequipment in the network, e.g., optical fiber amplifiers. A completelink failure occurs when all the fibers in the link are damaged orinhibited and data cannot pass through any of the fibers in the link. Inthe APS of the present invention, link or node failures are circumventedby re-routing signals from working fibers, which normally carry thedata, to protection fibers which are available to carry the data ifneeded, using protection switches in nodes at the ends of each networklink. The protection switches are activated immediately when a fault isdetected in the network.

The present invention uses redundancy (protection fibers) to improve thereliability performance of the data transport network. The power and/orBit Error Rate and/or line overhead information in the signal aremonitored at all the links and this information is used to determinewhen to switch a data path to the protection fibers. Generally, the timeit takes to detect a failure and switch to the protection fibers is onthe order of milliseconds, e.g. 60 msec. The protection switching isperformed at the individual switching nodes without an instruction froma central manager, and thus the APS process is distributed andautonomous. However, the APS switches do require some limited internalprotocol in order to coordinate the protection switching process for theentire network.

The link failures described above mean that both the main and protectionfibers in the link fail in both directions for that link. The objectiveof the automatic protection system is to ensure that a valid data pathwhich goes around the failed link is identified in a fast and efficientmanner. The data signals that would normally use the failed link arere-routed on a path made up of protection fibers, from the network node(or simply “node”) on one side of the failed link to the receivingnetwork node on the other side of the failed link. The APS process ofthe present invention will restore the failed link in both directions.However, the invention is directed to the more troublesome cases of acomplete link failure or failed node in the network.

FIG. 1A shows a planar network with seven network nodes and withbi-directional protection fibers accompanying each bi-directionalworking fiber. The network nodes 101, 103, 105, 107, 109, 111 and 113are selectively connected by working fibers (which are centered inlinks) indicated by the solid outer arrows between the nodes. The arrowsindicate the direction in which the data flows. In working fiber 115,data flows from network node 107 to network node 101 while in workingfiber 117 data flows in the reverse direction, from network node 101 tonetwork node 107. The protection fibers are shown by dotted arrows inthe network. The arrows also indicate the direction of the data flowthrough the fiber. For example, protection fiber 119 would transfer datafrom network node 101 to network node 107 if required. Additionally,protection fiber 121 would transfer data from network node 107 tonetwork node 101. As shown in network 100, each link between two networknodes contains two working fibers operating in opposite directions andtwo protection links operating in opposite directions. A link maycontain additional pairs of working and protection fibers.

Also shown in system 100 is an end user source S₁ 123 and acorresponding end user destination D 125. The data path between thesource and destination in the network 100 is shown operating without anylink or node failures by the bold line arrows 127 and 129. The dataflows from source S₁ 123 in network node 101 to working fiber 127 tonetwork node 105 to working fiber 129 to destination D₁ 125 in networknode 111. Also shown in system 100 is a second transmission route from asource S₂ 131 to destination D₂ 133. The data transmission route goesfrom source S₂ 131 in network node 111 to working fiber 135 to node 105to working fiber 137 to destination D₂ 133 in network node 101.

The invention protects against link failures for networks with planartopologies and networks with non-planar topologies. First we describethe protection against link failures for networks with planartopologies.

FIG. 1B shows a network node 99 connected to other network nodes in anetwork through four links. Each link includes a pair of working fiberstransmitting data in opposite directions. These working fibers areconnected to other network nodes not shown in FIG. 1B and a pair ofprotection fibers, transmitting data in opposite directions. Link 85includes working fiber pair 1, 3 and protection fiber pair 17, 19. Link87 includes working fiber pair 5, 7 and protection fiber pair 21, 23.Link 89 includes working fiber pair 9,11 and protection fiber pair 25,27. Link 91 includes working fiber pair 13, 15 and protection fiber pair29, 31. These protection fibers are also connected to other networknodes not shown in FIG. 1B. Within each link each working and protectionfiber pair transmitting data in opposite directions terminates at aprotection switch. In FIG. 1B, working fiber 1 and protection fiber pair17 in link 85 terminate at protection switch 33, working fiber 3 andprotection fiber 19 in link 85 terminate at protection switch 35,working fiber 5 and protection fiber 21 in link 87 terminate atprotection switch 37, working fiber 7 and protection fiber 23 in link 87terminate at protection switch 39, working fiber 9 and protection fiber25 in link 89 terminate at protection switch 41, working fiber 11 andprotection fiber 27 in link 89 terminate at protection switch 43,working fiber 13 and protection fiber 29 in link 91 terminate atprotection switch 45, and working fiber 15 and protection fiber 31 inlink 91 terminate at protection switch 47. All these working andprotection fiber pairs also terminate at protection switches at theother network nodes which are connected to network node 99 but not shownin FIG. 1B. In the preferred embodiment of an optical network,protection switches are opto-mechanical switches but could also be anytype of conventional switching devices depending upon the type ofnetwork being protected. The state of the protection switches in thenetwork under normal non-failure operation as shown by the protectionswitch position in FIG. 1B is termed the BAR state. When a failure isdetected, some of the protection switches are reconfiguredappropriately, as described below.

Network node 99 also contains a center switch 49 which routesinformation from its incoming to its outgoing ports. In the preferredembodiment of an optical network, center switch 49 can be implemented byan optical switch, an example of which is described by Iqbal et al. in“High Performance Optical Switches for Multiwavelength RearrangeableOptical Networks”, Government Microelectronic Circuits ApplicationConference (GOMAC) '94, San Diego, Calif., November 1994 (which ishereby incorporated by reference). They could also be any type ofconventional switching devices depending upon the type of network beingprotected. The center switch interconnection settings are in response tocommands from a network management entity.

While the network is operating free of failures, working fibers areconnected to center switch 49 through protection switches and workinginterconnection fibers. Working fiber 1 is connected to center switch 49through protection switch 33 and working interconnection fiber 51.Working fiber 3 is connected to center switch 49 through protectionswitch 35 and interconnection fiber 53. Working fiber 5 is connected tocenter switch 49 through protection switch 37 and interconnection fiber55. Working fiber 7 is connected to center switch 49 through protectionswitch 39 and interconnection fiber 57. Working fiber 9 is connected tocenter switch 49 through protection switch 41 and interconnection fiber59. Working fiber 11 is connected to center switch 49 through protectionswitch 43 and interconnection fiber 61. Working fiber 13 is connected tocenter switch 49 through protection switch 45 and interconnection fiber63. Working fiber 15 is connected to center switch 49 through protectionswitch 47 and interconnection fiber 65. Protection fibers areinterconnected among themselves through protection switches andprotection interconnection fibers. For example, protection fiber 17 isconnected to protection fiber 31 through protection switch 33,protection interconnection fiber 67 and protection switch 47. Protectionfiber 19 is connected to protection fiber 21 through protection switch35, protection switch 37 and protection interconnection fiber 69.Protection fiber 23 is connected to protection fiber 25 throughprotection switch 39, protection switch 41 and protectioninterconnection fiber 71. Protection fiber 27 is connected to protectionfiber 29 through protection switch 43, protection switch 45 andprotection interconnection fiber 73.

Transmitter 81 and receiver 83 are connected to center switch 49 throughaccess fibers 77 and 75, respectively. Other end user equipment can alsobe connected if desired.

FIG. 2A shows the connections between two network nodes which are joinedby a link before a link failure occurs. In both network nodes 2×2protection switches are used which can protect against link failuresonly. The protection switches are shown in the default BAR state. Largerprotection switches which can protect against center switch failures aswell, in networks with planar topologies, are shown in FIG. 8. In thepreferred embodiment of an optical network, network node 105 from FIG.2A includes an optical center switch 201 which routes data arriving orleaving network node 105 through working fibers 135 and 129,respectively. It also includes protection switch 203 for connectingworking interconnection fiber 221 to protection interconnection fiber223 if a link failure occurs, and protection switch 205 for connectingworking interconnection fiber 225 to protection interconnection fiber227 if a link failure occurs. Protection interconnection fibers 223,227, 235, 239 in network nodes 105 and 111 are connected to otherprotection switches in those network nodes (not shown). In the figures,“W” refers to working interconnection fibers “P” refers to protectioninterconnection fibers. Network node 105 also contains a pair ofprotection switches connected to fibers located on each of the threeother sides of the network node (not shown). Network node 111 containsan optical switch 207 which routes data arriving or leaving from networknode 111 through working fibers 129 and 135 respectively. It alsoincludes protection switch 209 for connecting working interconnectionfiber 231 to protection interconnection fiber 235 if a link failure(fault) occurs. Protection-switch 211 is included for connecting workinginterconnection fiber 237 to protection interconnection fiber 239 if alink failure occurs. Network node 111 also contains a pair of protectionswitches connected to fibers located on each of the three other sides ofthe network node (not shown). Each protection switch is assembled with asingle working and protection fiber incident to the node. The othernetwork nodes in FIG. 1A have the same type of connections as shown inFIG. 2A.

In the example of FIG. 2A, no link failure has occurred, data flowsbetween the network nodes along working fibers 129 and 135. Theprotection fibers are not utilized if there are no failures.

FIG. 2B shows the connections within the two network nodes 105 and 111of FIG. 2A after a link failure has occurred. The protection switchesare now shown in the cross state after the failure. The link failuretypically severs all four fibers 129, 135, 140 and 142. When the linkfailure is detected by the network nodes on either side of the link,e.g., by monitoring the power level of the signals on the workingfibers, and comparing it to an acceptable threshold, by line overheadinformation of the signal or by any other type of failure detectionsystem, the protection switches within the network nodes at both sidesof the link failure are set to the cross state, and automatically switchthe working fiber data flow onto the associated protectioninterconnection fibers which are oriented for data flow in the oppositedirection from the corresponding working (and working interconnection)fibers. In the example of FIG. 2B, protection switches 203, 205, 209 and211 can be opto-mechanically operated switches or can be any switchingmeans which operates to redirect the data flow in the network from aworking interconnection fiber onto a corresponding protectioninterconnection fiber and from a protection interconnection fiber onto acorresponding working interconnection fiber.

After a link failure occurs, protection switch 203 in FIG. 2B directsthe data that would normally flow over working fiber 129 from networknode 105 to network node 111 onto protection interconnection fiber 223in the direction away from the failed link. After the link failure,protection switch 205 in FIG. 2B also redirects the data so that workinginterconnection fiber 225, which normally receives data from networknode 111 along working fiber 135, now receives data from protectioninterconnection fiber 227. The automatic protection switching operationto bypass the failed link also occurs at network node 111 on the otherside of the failed link. After a link failure occurs, protection switch211 in FIG. 2B now directs the data that would normally flow overworking fiber 135 from network node Ill to network node 105 ontoprotection interconnection fiber 239 in the direction away from thefailed link. After the link failure, protection switch 209 in FIG. 2Balso redirects the data that would normally flow over working fiber 135from network node 111 to network node 105 onto protectioninterconnection fiber 239 in the direction away from the failed link.After the link failure, protection switch 209 in FIG. 2B also redirectsthe data. Working interconnection fiber 231 which normally receives datafrom network node 105 along working fiber 129, now receives data fromprotection interconnection fiber 235.

The result of the automatic protection switching described above is aredirection of data which would have been passed through the failedlink, onto a protection interconnection fiber which will automaticallyroute the data to the other side of the failed link along a protectionfiber path in a protection cycle. Additionally, the data which was toreach network node 111 through the failed link is now obtained from aprotection interconnection fiber 235 in the protection cycle containingthe rerouted data. The other direction is similarly protected throughanother protection cycle. Thus, the failed link is completely avoided inthe transfer of data and the network is restored very quickly to fulloperation. The failure is detected at the network nodes on both sides ofthe failed link, and the protection switching is performed within thosetwo network nodes. The protection cycles are made up of the protectionfibers and protection interconnection fibers and are pre-determined andset up prior to the activation of the network. No centralized decisionprocess as to which protection fibers must be used in the network toredirect the data in case of a link failure is necessary. This allowsfor fast, immediate protection of a link failure.

The protection fibers are organized into a number of protection cycleswithin the network. The term “protection cycle” as used in applicants'specification and claims is a closed loop of unidirectional fibers. Inthe present invention, each protection fiber is part of one and only oneprotection cycle in the automatic protection switching system. Also inthe invention, a protection cycle will not contain two protection fibersoriented in opposite directions in the same length unless the protectioncycle includes a bridge. A bridge occurs when a network node isconnected to the rest of the network through a single link. Furthermore,a bridge occurs when a link is the only point of connection betweenotherwise disconnected parts of a network. If the bridge is severed, thenetwork is cut into two disconnected parts, and there is no way totransmit data between those parts without restoring the link, becausethere is no alternate pass available.

The protection cycles, used to protect the network against a linkfailure, are predetermined by modeling the network as a directed graphwith vertices representing the network nodes and directed edgesrepresenting the protection fibers in the network. One embodiment of thepresent invention has a network with a planar topology, hence itscorresponding graph is also planar. A graph is planar by definition ifits edges can be drawn on a plane so that no two edges intersect at anyother point but a vertex. When so drawn, the graph is called a planegraph. FIG. 3 shows a plane graph representation of a network withplanar topology, with calculated directed cycles in accordance with theinvention. The vertices, representing network nodes 301, 303, 305, 307,309, 311, 313, 315, 317, 319, and 321 are shown by the small circles inFIG. 3. Directed cycles 323, 325, 327 and 329 are directionally orientedin the same manner, i.e., counter-clockwise, in the inner faces of theplane graph. An inner face of a plane graph is the inside of three ormore edges connected together to form a closed space. A directed cycle331 oriented in the opposite direction, clockwise in this example, isplaced around the outside of the planar graph (outer face).

After the appropriate directed cycles are computed in the directed graphrepresentation of the network, the protection switches in the networkare interconnected with protection interconnection fibers so that theresulting directed closed paths of protection fibers correspond to thedirection cycles found in the corresponding directed graph. For example,directed edges 333, 335, 337 and 339 in FIG. 3 form directed cycle 323.Directed edges 333, 335, 337 and 339 correspond to protection fibers inthe corresponding network. The protection switch in network node 311(modeled as vertex 311 in corresponding graph) that terminatesprotection fiber 333 is interconnected to the protection switch thatterminates protection fiber 335 also in network node 311. Thisinterconnection is made with a protection interconnection fiber.Similarly, the interconnections are made for the protection switchesterminating protection fibers 335 and 337 in network node 305, theprotection switches terminating protection fibers 337 and 339 in networknode 301, and the protection switches terminating protection fibers 339and 333 in network node 309, all of which create protection cycle 323.

This configuration of the automatic protection switching system achievesthe condition of the present invention of having two protection fibersoriented in opposite directions in different protection cycles for everylink between two network nodes, unless there is a bridge as previouslydefined. It also achieves the condition that each protection fiberappears in one and only one protection cycle. Vertices 303 and 321(representing network nodes) are attached to bridge links because theyare connected to the rest of the graph (representing the network) onlythrough one link. If the link that connects network node 303 to networknode 305 in the corresponding network is severed, it would not bepossible to transfer the necessary information in a protection cyclewithout repairing the faulty link. If a bridge link failure occurs inthe network, there can be no suitable protection for that bridge link.The network node on the other side of the bridge can only receive datafrom the bridge link itself. Therefore, a network should be designed tominimize the number of bridge links in order to gain maximum operationalprotection through the use of the protection cycles. If the net topologyis modified after initial network activities, through the addition ordeletion of network nodes or links, the protection cycles can easily bere-computed to give complete protection to the entire network.

FIG. 4 shows the planar network of FIG. 1A with one of the links betweenthe network nodes failing. The network shown has the same configurationas in FIG. 1A and like numbers are used in the description of FIG. 4.The protection switches in the network nodes are interconnected withprotection interconnection fibers to create five protection cycles aspreviously defined. Protection cycles in the network are configured andoriented in a clockwise direction for the inner faces of the network andan additional protection cycle is oriented counter-clockwise around theouter face of the network. One of the inner protection cycles is made ofprotection fibers 401, 403 and 405. The cycle is a complete data pathwhich allows transportation of data from any single network node in theprotection cycle to another network node in the protection cycle.Another of the inner protection cycles is made of protection fibers 409,411, 413 and 431. The other protection cycles in the network are shownas complete cycles of protection fiber and protection interconnectionfiber in a single direction indicated by the dashed line arrows.

A failure indicated by “X” 407 disrupts the connection for the fourfibers in the link, two working fibers and two protection fibers,located between network nodes 105 and 111. When the failure is detectedby the network nodes on both sides of the failure, the network nodesautomatically switch the data path from the working fibers to theprotection interconnection fibers, as previously shown in FIG. 2B. Afterthe switching operation is performed, the data path now established forthe transmission of data from source S₁ 123 to destination D₁ 125 isindicated by the combination of bolded working fibers (full line arrows)and bolded protection fibers (dashed line arrows). The data, after linkfailure protection is enacted, is transmitted from source S₁ 123 innetwork node 101 to working fiber 127 to network node 105 as transmittedbefore the failure. Now, because network node 105 has switched thetransmission path from the working fibers to the fibers in theprotection cycle at protection switch 450, the data is transmitted onprotection fiber 409 to network node 103 instead of onto working fiber129. The data path now follows the protection cycle in the clockwisedirection along protection fiber 411 to network node 109. The data thenflows to protection fiber 413 which arrives at network node 111 throughprotection switch 415. The data continues along the protection cyclewithin network node 111 and is directed to protection switch 417 in thenetwork node. Protection switch 417 has been activated when the linkfailure was detected by network node 111 to direct the data fromincoming protection interconnection fiber onto the outgoing workinginterconnection fiber to destination D₁ 125 in network node 111 as shownin FIG. 2. Therefore, data is delivered from source S₁ to destination D₁along the new data path including protection fibers just as if the linkfailure 407 had not occurred. The pre-calculated protection cyclesensure that the data will be delivered along a protection data path tothe receiving node on the other side of a failed link if a failure doesoccur.

The data from the source S₂ 131 in FIG. 4 is also transferred to itsdestination D₂ 133 using the protection cycles in the network in theevent of a link failure. Because of the link failure 407, the protectionswitches 417 and 418 have been activated which change the data path fromthe working interconnection fibers to the protection interconnectionfibers, and vice-versa, as shown in FIG. 2B. The data now flows fromsource S₂ 131 in network node 111 along protection fiber 419 to networknode 113. The data is then transferred along protection fiber 421, partof one of the protection cycles, to network node 107. The data is thentransferred along protection fiber 423 to network node 105. Due to theprotection switch 451 setting in network node 105, the data is nowtransferred back onto working fiber 137 and then to destination D₂ 133in network node 101. The protection cycle in the lower right quarter ofthe network allows for the proper transmission of the data for thissource to destination data transmission.

The protection cycle configuration of the present invention allows for amaximum restoration of └f/2┘ simultaneous link failures whenbi-directional connections are restored and a maximum restoration of(f−1) simultaneous link failures when uni-directional connections arerestored, where f represents the total number of faces when the networkis represented by a plane graph. The present invention guarantees arestoration of any single link failure (excluding bridges) and allowsfor possible restoration of multiple link failures depending on theposition of the link failures.

FIG. 5 shows a planar network with two link failures. The network nodesand protection cycles are configured in the same manner as in FIGS. 1and 4. In this example, a link failure “X” 519 has occurred and a linkfailure “X” 521 has occurred simultaneously in the network. When eachfailure occurs, the protection switches in the network nodes on bothsides of the failures are activated and the data to and from the workinginterconnection fibers are transferred to and from the protectioninterconnection fibers as shown in FIG. 2B. The new data path after bothlink failures is show in the bolded working fibers (full line arrows)and bolded protection fibers (dashed line arrows).

The data paths established after the two link failures will now bedescribed. After detection of the link failures, the protection switchesin the network nodes on both sides of each link failure are activated.The data from source S₁ 501 traveling to destination D₁ 503 travels fromsource S₁ 501 to network node 505 to working fiber 531 to network node509, in the same manner as before the link failure 521 occurred. Now,due to link failure 521 and the resulting activation of the protectionswitch 559 in network node 509, the data is transferred to protectionfiber 535. The data is then transferred to network node 507 toprotection fiber 557 to network node 513 to protection fiber 539 tonetwork node 515. The data follows a protection cycle around the failedlink to arrive at the network node on the other side of the failed linkand then on to the proper destination. The data is transferred off theprotection fibers in the protection cycle in network node 515 because ofthe activated switch 561. The data is finally transferred to destinationD₁ 503.

The data from source S₂ 523 would normally be transmitted to networknode 507 directly and then to network node 513. However, there is afailure indicated by “X” 519 in the link between network nodes 505 and507. Therefore, the protection switches in network nodes 505 and 507which are connected to the failed link are activated. The data is thentransferred onto protection fiber 541 from network node 505. The datawill be transferred along the protection cycle until it reaches theprotection switch that has been activated on the other side of thefailed link. The data travels from source S₂ 523 along the outside(outer face) protection cycle and is sent from protection fiber 541 tonetwork node 511 to protection fiber 543 to network node 517 toprotection fiber 545 to network node 515 to protection fiber 549 tonetwork node 513 to protection fiber 551 to network node 507. In networknode 507, the protection switch 563 adjacent to the failed link has beenactivated so the protection interconnection fiber is connected back tothe working interconnection fiber.

The data is then transferred from network node 507 to working fiber 553to network node 513 to destination D₂ 525. The data path from S₂ to D₂,after the link failures, passes through node 513 on its way to networknode 517 because the data path follows the protection cycle to thenetwork node on the other side of the failed link. The data was onlytransferred back to the working fibers at network node 507, which was onthe other side of the failed link. In a distributed automatic protectionswitching system, network node 513 is unaware of the link failure sincethe links surrounding network node 513 have not failed. This feature ofpre-assigned protection cycles allows for fast switching to a protectionroute and ensures a valid data path without requiring any centralizedprocessing to determine a protection route. The protection cycles ensurethat the data will arrive at the other side of the failed link andultimately to its destination.

Each network node can send one of three commands to its protectionswitches (via a network node controller located within the network nodeand connected to each of the protection switches) to achieve signal pathrestoration after a link failure. The network node can send a “clear”command, a “lockout protection” command or a “switch to protection”command. The “switch to protection” command is issued when a nodedetects the failure of an incident link and switches the data from theworking interconnection fibers onto the protection interconnectionfibers and vice-versa as shown in FIG. 2B. All other protection switchesremain in their default positions (BAR state). The protection switchingperformed at the network nodes adjacent to the failed link allows forthe data path to circumvent the failed link by reaching the network nodeon the other side of the failed link using the protection fibers in theappropriate protection cycle. Once the data reaches the network node atthe other side of the failed link, the data is switched back to theworking fibers to continue on its path to its assigned destination.

Once a failure occurs, a “lockout” protection command will preventfurther switching of the protection switches involved in restoring thatfailure. Otherwise, the original protection cycle may be interrupted inan attempt to repair a second failed link. After a link failure isrestored, the “clear” command is issued first for the protectionswitches in the two network nodes on both sides of the restored link,and subsequently to all protection switches in the protection cycle usedfor restoration, and the protection switches are returned to theirdefault (BAR) state and are not “locked.”

The above discussion focuses on networks with planar topologies;however, the inventive technique can be extended to networks withnon-planar topologies as well. A non-planar bridgeless graph has anorientable cycle double cover, which means that each bi-directional edgein a network will appear in exactly two directed cycles, one in eachdirection. A graph corresponding to a network (of any topology) hasbi-directional edges corresponding to the protection fibers in thenetwork. Each bi-directional edge consists of a pair of directed edgesin opposite direction. Thus said edges can be divided into directedcycles such that each directed edge appears in exactly one directedcycle and both directions of a single bi-directional edge does notappear in the same directed cycle. Therefore, protection cycles with thecharacteristics previously defined can always be found for networks withnon-planar topologies.

The directed cycles can be determined in a non-planar graph by applyinga conventional algorithm called backtracking (also called branch andbound algorithm) together with a novel algorithm that checks and ensuresthat the constraints imposed on the directed cycles discussed above arefollowed. Backtracking is well known in the art. (See e.g.,Combinatorial Algorithms, by T. C. Hu, Chapter 4, pages 138-161, AddisonWesley Publishing Company, 1982; which is hereby incorporated byreference). While backtracking can be computationally complex theprotection cycles can be determined before the network is in actual useso that a large computation time is not a concern. Other conventionalmethods of determining the directed cycles may also be used. Once theprotection cycles are in place, there is no need for furthercalculations in operating the automatic protection switching system.

A network configuration of network nodes and links can be tested todetermine if a network has a planar topology or not, by using a planartesting algorithm well known in the art. (See e.g., Algorithmic GraphTheory, by Alan Gibbons, page 85-93, Cambridge University Press, 1985;which is hereby incorporated by reference). Once a determination is madeof whether the network has a planar topology or not, the protectioncycles for that particular type of network can be found and implementedin the network.

The automatic protection switching system of the present invention canalso protect against switch failures in the network. In the preferredembodiment of an optical network, the center switch will be an opticalswitch. It could also be any type of conventional switching devicedepending upon the type of network being protected. In order to achievecenter switch protection, the protection switches in the network nodesmust be slightly modified.

FIGS. 6A and 6B show[s] the modified protection switches (located withina network node) which are used in this invention to protect againstcenter failure. In FIG. 6A, switch 601 is a 2×3 protection switch (twoinputs and three outputs). In FIG. 6B, switch 603 is a 3×2 protectionswitch (three inputs and two outputs). Switch 601 has a workinginterconnection fiber 605 input, a working fiber 607 output, a firstprotection fiber 611 input, a first protection interconnection fiber 613output and a second protection interconnection fiber 609 output.Protection switch 603 has a working fiber 621 input, a workinginterconnection fiber 623 output, a protection fiber 627 output, a firstprotection interconnection fiber 625 input and a second protection fiber630 input. The protection switches 601 and 603 shown in FIGS. 6A and 6B,respectively, are placed in the same manner within the network node asthe 2×2 protection switches used only for link failure protection. Theonly difference is that there are now some additional protectioninterconnection fiber connections among the protection switches in thenetwork nodes. There are three different possible switch states for each3×2 protection switch. The first switch state is “BAR-1” which occurs inthe default state and connects the protection fiber to the firstprotection interconnection fiber and the working fiber to the workinginterconnection fiber. For example, in switch 601 shown in FIG. 6A,protection fiber 611 is connected to first protection interconnectionfiber 613 and working fiber 607 is connected to working interconnectionfiber 605. The second switch state “BAR-2” connects the protection fiberto the second interconnection fiber and the working fiber to the workinginterconnection fiber. For example, protection fiber 611 is connected tofirst protection interconnection fiber 609 and working fiber 607 isconnected to working interconnection fiber 605. The third switch stateis the “Cross” state where the protection fiber is connected to theworking fiber and the working interconnected fiber is connected to thefirst protection interconnection fiber. For example, protection fiber611 is connected to first working fiber 607 and working interconnectionfiber 605 is connected to first protection interconnection fiber 613.

In the present invention, only center switch failures in networks withplanar topologies are discussed. Out of all connections going throughthe “failed” network node, only a single priority connection can berestored. Priorities are set up for all connections in the network andthe “failed” network node will switch the protection switchesappropriately so that the highest priority connection passing through itis restored. The approach is the same as for the link failures in planarnetworks. The planar network is modeled as a planar graph; it is thendrawn on the plane and the inner and outer faces are found. The directedcycles are then formed as described previously. The only difference isthat in this case the first protection interconnection fibers are usedto create the protection cycles after the corresponding directed cyclesare determined. The method to do this is the same as explained above.The second protection interconnection fibers are used to interconnectthe protection switches terminating the protection fibers contained inthe same link.

Each network node in the network can detect a center switch failure byinternal or external monitors. Any monitoring system, e.g., power lossdetection, can be used for detection of the center switch failuredepending on the type of network center switch being protected.

In the event of a center switch failure, all of the protection switcheswithin the network node containing the center switch that failed(“failed network node”) will be switched. The protection switches withterminating working fibers on the priority path are switched from BARstate to Cross state while all other protection switches in that networknode are switched from the BAR-1 state to the BAR-2 state. Theconfiguration ensures that the failure at the center switch will becircumvented because the data will flow along the second protectioninterconnection fibers while within the “failed” network node and alongthe protection cycles outside the “failed” network node until they reachthe priority working fiber on the other side of the “failed” networknode.

FIG. 7 shows the configuration of the protection cycles after centerswitch 701 failed and the protection switches in the network node wereactivated. FIG. 7 also shows a number of protection switches in thenetwork node which interface with the incoming and outgoing fibers. Forexample, protection switch 703 has a working fiber 709 as an input and aprotection fiber 705 as an output. The configuration of the protectioncycles in this example corresponds to the restoration of priorityconnection consisting of working fibers 702, 704, 709 and 711. After thecenter switch fails, the signal from working fiber 709 is placed on theprotection fiber 705 to be transported around the “failed” network nodeto the priority path on the other side of the “failed” network node. Ifthe data happens to return to the “failed” network node (as is the casein this example) and that particular path is not the priority path, thedata enters and exists the “failed” network node via the secondprotection cycle. Eventually the data will reach the priority path onthe other side of the “failed” network node and be switched back on thecorrect working fiber.

FIG. 8 shows the connections within the network node 801 after a centerswitch failure has occurred in network node 801. The center switchfailure incapacitates the center switch which can no longer be used forrouting information in the network. FIG. 8 shows network node 801 withthree sets of both protection switches 601 and protection switches 603,where each pair is associated with a particular connection direction inthis example (west, north and 25 east). The center switch 803 of networknode 801, which connects the data between the protection switchesoriented in different directions has failed as indicated by the “X” 805.When a center switch failure is detected inside the network node, thenetwork node checks the priorities for all connections going through andfinds the connection with the highest priority. For this example,working fibers 811, 825, 827 and 829 are used in the highest priorityconnection. The protection switches 855, 857, 859 and 867 terminatingthe fibers used for the priority connection will then switch from theBAR-1 to the Cross state in order to prevent the data from flowing intothe failed center switch. Protection switches 863 and 865, notterminating the fibers used for the priority connection will switch fromthe BAR-1 state to the BAR-2 state. In this example, where priority datais coming into the “failed” network node on working fiber 811, it isswitched onto the protection fiber 813. The protection cycles are 831,833 and 835 with the orientations shown in FIG. 8 so, the data is thentransmitted along protection cycle 831 and is placed onto protectionfiber 815 which comes into protection switch 863 at the north side (topof Figure) of the network node. The second protection interconnectionfiber 817 then directs the data from protection switch 863 to protectionswitch 865 at the north side of the network node. The data is thenplaced on another protection cycle 833 through protection fiber 819.After following protection cycle 833, the data returns to the “failed”network node via protection fiber 823 into protection switch 859 at theeast side of the node. The protection switch 859 has been previouslyactivated to connect the protection fiber to the working fiber to allowthe priority data to continue through the network on to its destination.Instead of traveling through the “failed” network node, the data reachedits proper destination going around the failed center switch of thenetwork node using the protection cycles. Network node 801 also allowsthe designated priority data to travel in the opposite direction. Asimilar path is followed by the priority data flowing in the oppositedirection which uses protection cycles which were not used for the firstpriority data path around the failure.

If there was not a center switch failure, but a link failure instead,then the second protection interconnection fibers would not be utilizedand all connections through the failed link would again be restored. Forexample, in FIG. 8 if network link 871 fails, protection switches 85Sand 857 change from the BAR-1 state to Cross state while the otherprotection switch in the node remains in the BAR-1 state.

FIG. 9 shows a planar network with seven network nodes after a singleswitch failure. Network 900 includes network nodes 909, 911, 913, 915,917, 919 and 921. A failure in the optical switch in network node 913 isshown by the “X” 924. A data path needs to be established from source S,901 to destination D₁ 903. A second data path needs to be establishedfrom source S₂ 905 to destination D₂ 907. If a center switch failure hadnot occurred, the data from S₁ 901 in network node 909 would travel tonetwork node 913 to destination D₁ 903 in network node 919 as the mostdirect route. However, because of the failure in the center switch ofnetwork node 913, the data cannot pass through network node 913. Whenthe failure occurs, the priority bi-direction data path is selected(from all data paths traveling through the network node) and thoseprotection switches along that path in the “failed” network node areactivated to switch data from the working fibers to the protectionfibers and vice-versa. All other protection switches in the “failed”network node are switched from the first protection interconnectionfibers to the second protection interconnection fibers. This allows thedata traveling over the priority path to avoid the “failed” network node(will not use the failed center switch in that network node) by beingswitched to the protection cycles to travel around the network node withthe failed center switch.

FIG. 9 shows the setting on the protection switches after the centerswitch failure has been detected. The data from source S₁ 901 in thenetwork node 909 travels along working fiber 928. Protection switch 950of network node 913 has been activated such that the data is now placedon protection fiber 930. The data that flows, from protection fiber 930through network node 909 and onto protection fiber 932 following thepre-designated protection cycle (shown by dashed bolded arrows).Protection switch 952 is activated such that the data is transferredonto second protection interconnection fiber 953 within network node 913which avoids the failed center switch. The data then travels throughnetwork node 913 and onto protection fiber 936. The data is nowfollowing a second protection cycle (also shown by dashed boldedarrows). The data travels through network node 911 onto protection fiber938 and then through network node 917 onto protection fiber 940. Thedata then flows through network node 919 onto protection fiber 942 tocomplete the protection cycle. Protection switch 955 has been activatedsuch that the protection fibers are re-connected to the working fibersbecause the destination at the other side of the “failed” network nodehas been reached. The data then continues on with its normal path alongworking fiber 944 to destination D₁ 903 in network node 919.

The only protection switches that were activated in the entire networkwere in network node 913. All the other network nodes did not have toactivate any protection switch nor did any centralized intelligencebecome necessary. The data simply followed the pre-assigned protectioncycles it was placed on. This is a important benefit to the APS systemof the present invention which does not depend on the connection stateof the network when the failure occurred.

The activated protection switches in “failed” network node 913 alsoallow the data to travel around network node 913 is the oppositedirection, from S₂ 905 to destination D₂ 907. The data travels fromsource S₂ to network node 919 to network node 913 where it follows twoprotection cycles to eventually arrive at destination D₂ in network node909.

The protection cycle configuration of the present invention allows for amaximum restoration of └f/2┘ simultaneous center switch failures whenbi-directional connections are restored and a maximum restoration of(f−1) simultaneous center switch failures when uni-directionalconnections are restored, when f represents the total number of faceswhen the network is represented by a plane directed graph. The presentinvention guarantees restoration of a single priority connection afterany single center switch failure, and allows for possible restoration ofsimultaneous multiple center switch failures (restoring one priorityconnection per failure) depending on the position of the failures.

Each network node now sends one of four commands to its protectionswitches (via a network node controller) to achieve signal pathrestoration for a priority connection after a center switch failure. Thenetwork node can send a “clear” command, a “lock protection” command, a“switch to protection” command (to a Cross state) and a “switch tosecond protection” command (to a BAR-2 state). The “switch toprotection” command is issued when the network node detects a centerswitch failure to the protection switch to a priority path. The “switchto second protection” command is also issued when the network nodedetects a center switch failure to the rest of the protection switchesin the node. All other protection switches in the network remain in thedefault state (BAR-1 state). The protection switching performed at thenetwork node where the center switch failed allows the priority datapath to circumvent the failed center switch by reaching the prioritypath on the other side of the “failed” network node using the protectionfibers in the appropriate protection cycles. Once the data reaches theother side of the network node in question, the data is switched back tothe working fibers to continue on its path to its assigned destination.

Once a failure occurs, a “lockout protection” command will preventfurther switching from a working to protection port or vice-versa andfrom a first protection interconnection port to a second protectioninterconnection port. Otherwise, the original protection cycle(s)required for priority data restoration may be interrupted in an attemptto repair a second failure. After a center switch failure is restored,the “clear” command is issued first for the protection switches in the“failed” network node, and subsequently to all protection switches inthe protection cycle(s) used for restoration, and the protectionswitches are returned to their default (BAR-1) state and are not“locked.”

FIG. 10 shows the steps for a method of providing an automaticprotection switching system for a network. Step 1001 models the networkcomprised of network nodes and links as a directed graph with verticesand edges corresponding to the network nodes and protection fibers,respectively. Step 1002 runs a conventional testing algorithm to decidewhether the network has a planar topology or not.

If the network is planar as determined in step 1002 the following stepsapply: Step 1003 draws the planar graph on the plane to obtain a planegraph with inner faces and an outer face. Step 1005 forms directedcycles with a chosen orientation for each inner face of the graph. Alldirected cycles along the inner faces of the graph will be oriented inthe same direction, clockwise or counter-clockwise. Step 1007 forms adirected cycle in the opposite direction than the chosen orientationalong the outer face. Step 1009 interconnects the protection switchesterminating the protection fibers with protection interconnection fibersas previously described to obtain protection cycles corresponding to thedirected cycles found in the graph representation of the network.

Steps 1001, 1002, 1003, 1005, 1007, 1009 are all performed prior to theactual operation of the network and therefore the time it takes todetermine the protection cycles does not affect the network's responsetime and is not a constraint on adequately protecting the network. Thesesteps will preferably be performed on a conventional computer, or insimple networks will be done manually.

Step 1011 switches the working interconnection fibers onto theprotection interconnection fibers and vice-versa if a link failureoccurs as described in FIG. 2B. This allows the data to be carried bythe protection fibers in the protection cycles to the network node onthe other side of the link failure without signaling or alerting anynetwork node other than the network nodes on both sides of the linkfailure. Because the automatic protection switching system can be set upbefore the activation of the network and because its configurationdoesn't depend on the active connection in the network, no additionaldelays or communications are necessary to remedy the link failure untilthe link is repaired.

Step 1013 protects against center switch failures using the 2×3 and 3×2protection switches as described with FIG. 6. If node failure protectionis not desired, the step will be skipped. First protectioninterconnection fibers are now used to create the protection cycles andsecond protection interconnection fibers are used to interconnect theprotection switches terminating the protection fibers belonging in thesame link. The data will now follow the protection cycles until itreaches the priority path on the other side of the “failed” network nodewhere it is switched back to the working fiber and continues to itsdestination. Previous protection attempts could not compensate for acenter unit failure without drastic rerouting of all data paths aroundthe node. The automatic protection switching system of the presentinvention allows the protection cycles to be set up before the network'soperation and its configuration doesn't depend on the active connectionsin the network.

If the network is not planar as determined in Step 102 the followingsteps apply: Step 1015 applies a suitable technique, such as abacktracking technique, to obtain the directed cycles with the specialcharacteristics mentioned above. Step 1017 interconnects the protectionswitches terminating the protection fibers with protectioninterconnection fibers as described above as to obtain protection cyclescorresponding to the directed cycles found in the graph representationof the network.

Steps 1015 and 1017 are also performed prior to the actual operation ofthe network and therefore the time it takes to determine the protectioncycles does not affect the network's response time and is not aconstraint on adequately protecting the network. The steps willpreferably be performed on a conventional computer, or in simplenetworks will be done manually.

Step 1019 switches the working interconnection fibers onto theprotection interconnection fibers and vice-versa if a link failureoccurs as described in FIG. 2B. This allows the data to be carried bythe protection fibers in the protection cycles to the network node onthe other side of the link failure.

An automatic protection switching system for a network can also beprovided for a network with a bridge. The bridge is simply disconnectedin the modeled network since the bridge cannot be restored as previouslydescribed.

The foregoing merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise numerous systems, apparatus and methods which, although notexplicitly shown or described herein, embody the principles of theinvention and are thus within the spirit and scope of the invention asdefined by its claims.

What is claimed is:
 1. An automatic protection switching system for anetwork comprising; a plurality of nodes; a plurality of pairs of firstand second primary conduits connected between said nodes wherein eachsaid first primary conduit's data flows in a direction opposite of saidsecond primary conduit in said pair; and a plurality of pairs of firstand second protection conduits connected between said nodes, whereineach said protection conduit is oriented to form one and only onepredetermined protection cycle of at least three predeterminedprotection cycles.
 2. The system of claim 1, wherein said conduits areoptical fibers.
 3. The system of claim 2, wherein each said nodecomprises optical switches.
 4. The system of claim 1, wherein said nodescomprise protection switches and said transmission direction of saidprimary and protection conduits is predetermined for said system.
 5. Thesystem of claim 4, wherein at least one said protection switchesconnects at least one said primary conduit to at least one saidprotection conduit when a failure occurs in one of said primary conduitsconnected to said node.
 6. The system of claim 1, wherein at least onenode further includes a means for detecting a failure within a nodeconnecting said primary conduit.
 7. The system of claim 6, wherein saidat least one node connects at least one said primary conduit in saidnode to at least one said protection conduit responsive to saiddetection means.
 8. The system of claim 1, further including at leastone third protection conduit within at least one node in said system. 9.The system of claim 8, wherein said at least one node with a said thirdprotection conduit further includes a means for detecting a failurewithin said node.
 10. The system of claim 9, wherein said at least onenode connects respective protection conduits that are part of twoprotection cycles using one said third protection conduit responsive tosaid detection means.
 11. The system of claim 1, wherein said network isa non-planar network.
 12. An automatic protection switching system forinsuring a working data pass in a network comprising: A plurality ofnodes; and a plurality of links wherein said links comprise a first andsecond primarily conduit connected between two of said nodes and a firstand second protection conduit connected between two of said nodes,wherein each of said protection conduits is oriented to form one andonly one of at least three predetermined protection cycles a respectiveone of which can become part of said data path if at least one faultoccurs in said network, and wherein said system is capable of ensuring aworking data path in said network if two or more said faults occur. 13.The system of claim 12, wherein said at least one fault includes a faultin one of said conduits.
 14. The system of claim 12, wherein said atleast one fault includes a fault in one of said nodes.
 15. The system ofclaim 12, wherein said conduits are optical fibers.
 16. The system ofclaim 15, wherein each said node comprises optical switches.
 17. Thesystem of claim 12, wherein said nodes comprise switching stations andsaid primary and protection conduits' said orientation is predeterminedfor said system.
 18. A method for providing an automated protectionsystem for a network which comprises nodes and connecting links,comprising the steps of: modeling said nodes and connecting links insaid network as a graph, wherein each said link comprises at least oneworking conduit and at least one corresponding protection conduit;forming a protection cycle in a predetermined direction for each insideface in said graph; forming a protection cycle in an opposite directionto said predetermined direction along said graph's outside face, whereinthere are formed at least three predetermined protection cycles;orienting each said protection conduit in the network to form one andonly one of the predetermined protection cycles; and switching a datapath from at least one said working conduit in a first said node withone of said corresponding protection conduits in said first node if afailure occurs in one of said links connected to said first node. 19.The method of claim 18, wherein said switching step connects said atleast one working conduit to said one of said corresponding protectionconduit oriented in the opposite direction to said at least one workingconduit.
 20. The method of claim 18, further including the step ofswitching a data path from a first protection conduit that is part offirst predetermined protection cycle to a second protection conduit thatis part of a second predetermined protection cycle using a thirdprotection conduit within a second node if a failure occurs within saidnode.